It has long been desired to realize such interactive control that buttons appear on a display screen as a video stream is reproduced and that the reproduction proceeds in accordance with user operations made to the buttons. A DVD is a breakthrough recording medium realizing such reproduction control. The synchronous presentation of buttons with a video stream is established through the use of time stamps set to cause the buttons to appear at specific points on the reproduction time axis of the video stream.
In order to realize the interactive control, however, it is not sufficient to record onto a recording medium graphics data used for rendering the buttons. A reproduction apparatus needs to be controlled so as to change the state of each button displayed on a screen, according to a user operation or to the proceeding of video stream reproduction. To realize the interactive control, a stream into which audio and video streams are multiplexed (Video Objects) is recorded on a DVD, and a NAVI pack containing state control information is provided at the beginning of each VOBU. The VOBU includes one GOP of the video stream, and also includes audio data and graphics data, which is supplemental to the video stream, both to be read simultaneously with the GOP from the DVD. The state control information is used to change, according to a user operation, the state of each button displayed on the screen. The NAVI pack contains information defining a transfer rate and a buffer size that each stream requires in handling the GOP. Since the DVD stores the state control information in NAVI packs, the button states can be changed with the time accuracy of GOPs. FIG. 1 illustrates the interactive control described above. In the figure, the lowest level shows data allocation on the DVD. It is shown that state control information is contained in a NAVI pack. The state control information remains valid during a time period of the GOP to which the NAVI pack belongs. Each graphics object is contained in a PES packet, and displayed at the same timing with a picture to be synchronized with the graphics object. This prior art technique is disclosed for example in JP patent No. 2813245.
It should be noted here that in DVD authoring, the structures of GOPs and VOBUs are not determined until elementary streams of video, audio, and graphics data are encoded and ready to be multiplexed into one VOB. The multiplexing is a final stage of authoring. That is, the state control information cannot be incorporated into a VOB before this final stage, and thus testing of how the buttons change on a display screen can not be carried out any earlier. Consequently, it is often the case that bugs are found right before the shipment and the developers are forced to make corrections in a rush. In addition, without enough time for the testing, it is risky to incorporate buttons of complex animation into a movie. For the reasons stated above, it is common in current authoring to incorporate relatively simple buttons, such as one which changes in color in response to a user operation.